Method and apparatus for sensing current and voltage in circuits with voltage across an LED

ABSTRACT

Methods and apparatuses to sense current and voltage in circuits with voltage drop. A sense circuit includes a series combination of a diode and a resistor in the path of the current to be sensed, the voltage across this series combination coupled to drive a light emitting diode (LED).

RELATED APPLICATION

This application claims priority to U.S. provisional application Ser.No. 60/306,719, filed Jul. 20, 2001, entitled “Method And Apparatus ForLow Cost Current And Voltage Sense Circuits With Voltage Drop.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to circuits with voltage andcurrent sensing and, more specifically, the present invention relates tocurrent and voltage sensing in circuits with voltage drop.

2. Background Information

Most battery operated portable electronic products such as cell phones,personal digital assistants (PDAs), etc. require a low power alternatingcurrent (AC) to direct current (DC) charger power supply with a constantvoltage and constant current (CC/CV) characteristics for chargingbatteries. Most of these chargers require relatively accurate andexpensive circuitry to meet the specified current and voltage tolerancesover temperature.

In a known circuit, voltage is sensed by using an accurate programmablereference IC such as a TL431, which drives an opto-coupler feedbackcircuit to control the output voltage at a programmed value set byexternal resistors. A relatively high level of accuracy is needed at theoutput of the charger circuit in order to meet a lower accuracy at theload due to voltage drop in the output cable that connects the chargerto the load (the electronic product). The voltage drop in the outputcable reduces the output voltage as load increases degrading the overallvoltage tolerance at the load. A required level of voltage accuracy atthe charger output can be achieved by choosing a TL431 IC that has beentrimmed to the appropriate accuracy level. TL431s with 3%, 2% and 1%accuracy are widely available. TL431 voltage reference is generally moreexpensive than a simple zener reference. However, zeners are generallydifficult to get at tolerance below 2% and the zener voltage varies withthe current through it, resulting in a poorer load regulation incircuits that have zener currents that vary with output load due to lowgain of the feedback loop. In addition, they are only available incertain standard voltage values, which makes it difficult to center theoutput voltage at the optimum point for the best tolerance.

For low cost, the current sensing in low power applications (e.g. <5W)is usually done by using a voltage drop across a current sense resistorto turn on a bipolar a transistor. This circuit uses the base emittervoltage, V_(BE) of the transistor as a reference. The transistor in turndrives an optocoupler feedback circuit to control the output current ata constant value. The constant current limit set by such a circuit,however, has a large temperature variation due to the high temperaturecoefficient of V_(BE) (−2 mV/° C.). This can be compensated to the firstorder by using a thermister based resistor network which add to thecomponent count and cost.

SUMMARY OF THE INVENTION

Methods and apparatuses for sensing current and voltage in circuits withvoltage drop are disclosed. In one aspect of the invention, a currentsense circuit is described including a series combination of a diode anda resistor in the path of the current to be sensed, the voltage acrossthis series combination coupled to drive an light emitting diode (LED).In one embodiment, the LED is part of an optocoupler and the optocoupleris part of a feedback circuit of a switched mode power supply. In oneembodiment, the diode is a PN junction diode.

In another aspect of the invention, a current and voltage sense circuitis described including a current sense circuit having a seriescombination of a diode and a resistor in the path of a load currentsupplied from an output and the voltage across this series combinationis coupled to drive an LED. The current and voltage sense circuit alsoincludes a voltage sense circuit coupled to the output. The voltagesense circuit includes a voltage reference coupled to the base of abipolar transistor. The bipolar transistor drives the LED when thevoltage at the output exceeds a sense voltage of the voltage sensecircuit. The voltage reference is a zener and the sense voltage of thevoltage sense circuit is the sum of the voltage across the zener and theforward base emitter voltage (V_(BE)) of the bipolar transistor. In oneembodiment, the LED is part of an optocoupler and the optocoupler ispart of a feedback circuit of switched mode power supply. In oneembodiment, the diode is a PN junction diode.

In yet another aspect of the invention, a voltage drop compensationcircuit is described including a voltage sense circuit coupled across avoltage output. The voltage sense circuit includes a voltage referencecoupled to drive the base of the bipolar transistor when the voltage atthe voltage output exceeds a sense voltage of the voltage sense circuit.A compensation resistor is coupled to the voltage output to carry acurrent that substantially represents the current that flows from thevoltage output to a load that is coupled to the output. The sum of thevoltage across the compensation resistor and the forward base emittervoltage of the bipolar transistor is applied across a series combinationof a second resistor and a diode. The current through the secondresistor is used to alter the sense voltage of the voltage sensecircuit. In one embodiment, the sense voltage is increased as thecurrent to the load increases. In one embodiment, the diode is replacedby a short circuit. In one embodiment, the voltage reference is a zenerand the sense voltage of the voltage sense circuit is the sum of thevoltage across the zener and the forward base emitter voltage (V_(BE))of the bipolar transistor and, the current through the second resistoris passed through the zener. In one embodiment, the voltage sensecircuit further includes a third resistor and the sense voltage is thesum of the voltage across the voltage reference, the voltage across thethird resistor and the forward emitter bias voltage of the bipolartransistor and, the current through the second resistor is passedthrough the third resistor. In one embodiment, the diode is a PNjunction diode. In one embodiment, the bipolar transistor drives an LEDof an optocoupler and the optocoupler is part of a feedback circuit ofswitched mode power supply.

In still another aspect of the present invention, a voltage dropcompensation circuit is described, which includes a voltage sensecircuit coupled across a voltage output. The voltage sense circuitincludes a first resistor coupled to a zener, which is coupled to drivean LED of an optocoupler when the voltage at the voltage output exceedsa sense voltage of the voltage sense circuit. The sense voltage of thevoltage sense circuit is the sum of the voltage across the zener and theforward voltage of the optocoupler LED and the voltage across the firstresistor. The voltage sense circuit also includes a current sensecircuit, which provides a voltage representative of current delivered toa load coupled to the voltage output. The voltage sense circuit alsoincludes a voltage compensation circuit coupled to change the voltageacross the first resistor of the voltage sense circuit responsive to thevoltage provided by the current sense circuit. In one embodiment, thevoltage across the first resistor is increased as the current suppliedto the load increases. In one embodiment, the current sense circuitincludes a series combination of a second resistor and a diode. In oneembodiment, the voltage compensation circuit includes a third resistorcoupled to the emitter of a bipolar transistor and a series combinationof the third resistor and a base emitter junction of the bipolartransistor is coupled across the current sense circuit, such that thecollector current of the transistor is proportional to the voltageacross the second resistor. In one embodiment, the bipolar transistorcollector is coupled to the first resistor such that the voltage dropacross the first resistor is responsive to the voltage across the secondresistor. In one embodiment, the optocoupler is part of a feedbackcircuit of switched mode power supply. Additional features and benefitsof the present invention will become apparent from the detaileddescription and figures set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention detailed is illustrated by way of example and notlimitation in the accompanying figures.

FIG. 1 shows one embodiment of a switched mode AC to DC power supplywith CC/CV output coupled to a load through a cable in accordance withthe teachings of the present invention.

FIG. 2 shows one embodiment of a current and voltage sense circuit withvoltage drop compensation in accordance with the teachings of thepresent invention used in the power supply of FIG. 1. in accordance withthe teachings of the present invention.

FIG. 3 shows one embodiment of the current vs. voltage characteristicsat the load in accordance with the teachings of the present invention.

FIG. 4 shows another embodiment of a current and voltage sense circuitwith voltage drop compensation in accordance with teachings of thepresent invention.

FIG. 5 shows another embodiment of a current and voltage sense circuitwith voltage drop compensation in accordance with teachings of thepresent invention.

DETAILED DESCRIPTION

Embodiments of methods and apparatuses for providing current sensing,voltage sensing and voltage drop compensation in circuits, such as forexample but not limited to power supply circuits, are disclosed. In thefollowing description, numerous specific details are set forth in orderto provide a thorough understanding of the present invention. It will beapparent, however, to one having ordinary skill in the art that thespecific detail need not be employed to practice the present invention.In other instances, well-known materials or methods have not beendescribed in detail in order to avoid obscuring the present invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, the appearances of thephrases “in one embodiment” or “in an embodiment” in various placesthroughout this specification are not necessarily all referring to thesame embodiment. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments.

As an overview, embodiments of the present invention provide a very lowcost, constant voltage, constant current sensing circuit that isrelatively temperature independent. A cable resistance compensationcircuit in accordance with one embodiment increases accuracy of thevoltage at the load by compensating for the cable voltage drop. Thisalso allows a lower cost voltage sense circuit to be used. A currentsense circuit according to one embodiment can also be used for detectingthe sensed current crossing a threshold for monitoring and protectionpurposes. One embodiment the present invention can be used to provide arelatively accurate, temperature compensated constant voltage andcurrent characteristic for battery chargers.

One of the described embodiments of the present invention uses zenerbased voltage sense for low cost. The reduced accuracy of the zenersensing is compensated for by a novel voltage drop compensationtechnique. This technique increases the sense voltage as a function ofload to compensate for cable voltage drop and any change in zenervoltage due to load related change in zener current. The net effectivevoltage accuracy measured at the load with this circuit is comparable tomore expensive TL431 based circuits. Alternatively, the voltage dropcompensation circuit could also be used in combination with moreaccurate voltage sense circuits such as the ones based on TL431 IC, tocompensate for resistive drops in the conductors (wires, cables, printedcircuit board traces, etc.) that connect the load to the voltage sensepoint, to further improve regulation accuracy at the load.

In one embodiment, the current sensing is done by using the forwardvoltage of the Light Emitting Diode (LED) as a voltage reference. Thecurrent is sensed by a series combination of a PN junction diode and aresistor and the voltage developed by this series combination is appliedacross the LED. This circuit takes advantage of the fact that the LEDforward voltage has a temperature coefficient that is similar to that ofthe PN junction diode, but its absolute value is approximately 300 mVhigher than that of the PN junction diode; i.e. a forward voltage ofapproximately 1V for LED vs. 0.7V for the PN junction diode. Thus, thevoltage across the sense resistor in one embodiment is 300 mV and isrelatively independent of temperature. In this embodiment, this currentsense circuit provides a relatively temperature independent constantcurrent output using LED of an optocoupler in a closed loop feedbackcircuit. An added advantage of this circuit is that it provides constantcurrent all the way down to a short circuit at the output. This isbecause the optocoupler LED is always driven in the constant currentmode, independent of the output voltage.

In other embodiments, the simple low cost current sense circuitdisclosed can also be used in an open loop fashion to detect a currentcrossing a threshold for monitoring and protection purposes, such as,over-current or overload detection, under-current or light loaddetection, over current shutdown, etc. In these applications, thecrossing of current threshold can be detected by the turn on (whencurrent cross above the threshold) and turn off (when current crossesbelow the threshold) of the output transistor of the optocoupler.Alternatively, a stand alone LED can be used to visually indicate acurrent crossing a threshold. For example, the LED on a power supplycould light up to indicate that a power supply is loaded or over loaded.

Thus, embodiments of the presently described current sense circuit, arelatively temperature independent current threshold is obtained with acircuit that is very simple and low cost. It is appreciated thatalthough embodiments of the present invention are described herein witha switched mode power supply, it is noted that other embodiments of thepresent invention are not limited in use to switched mode power suppliesor even to power supplies in general. For example, the embodiments ofthe present invention can be used for sensing voltage and current inlinear power supplies, instrumentation systems that have remote sensors,monitoring of current or voltage crossing a set threshold, etc.Embodiments of the present invention can be used in a open loop systemfor monitoring voltage or current or in a closed loop system forregulation of voltage or current.

FIG. 1 shows a switched mode AC to DC power supply 101 with CC/CV outputcoupled to a load 103 through a cable 105 having resistances of RC1 107and RC2 109 in the positive and negative conductors respectively. Thiscircuit uses one embodiment of the novel current sense, voltage senseand voltage drop compensation circuit according to the teachings of thepresent invention (shown within dotted lines) on the secondary side 129of the transformer T1 111. The circuit operates in closed loop, toprovide a relatively accurate constant current, constant voltage output.

In one embodiment, R1 113 is a fusible resistor that acts like a fusefor protecting the power supply 101 from component shorts and overloads.RV1 115 is a metal oxide varistor (MOV) used to clamp AC line transientfrom damaging the power supply. Diodes D1-D4 form a rectifier bridge 117to convert AC to DC and the capacitors C1 119 and C2 121 smooth the DCvoltage. C1 119 and C2 121 in combination with inductor L1 123 alsoprovide electromagnetic interference (EMI) filtering for high frequencynoise that is generated by the switching power supply 101 to limitconducted emission transmitted on to the AC line. U1 125 is a switchedmode power supply controller with an integrated high voltage switch. Inone embodiment, U1 125 is a TNY264 device, which is a member ofTINYSwitch II family of switched mode power supply ICs manufactured byPower Integrations of San Jose, Calif. Power supply controller 125converts the high voltage DC across C2 121 into high frequency AC acrossthe primary winding 127 (terminals 1 and 4) of the transformer T1 111.The amount of power delivered to the secondary terminals 129 (8 & 10) ofT1 111 is controlled in closed loop through the EN/UV pin of the powersupply controller 125. An optocoupler U2 131 is used to provide thefeedback with isolation from the secondary side 129 of transformer 111for closed loop control. In one embodiment, elements R1 133, R2 135, D5137 and C3 139 clamp leakage inductance spikes across the primarywinding 127 of transformer T1 111. C4 141 is a bypass capacitor for thepower supply controller 125 internal supply pin. D6 143 providesrectification and C5 145 provide filtering on the secondary 129 todeliver a DC voltage across C5 145. The circuit enclosed by the dottedline box in FIG. 1 on the secondary side 129 of transformer T1 111 isone embodiment of a current sense, voltage sense and voltage dropcompensation circuit 147 in accordance with the teachings of the presentinvention that is shown separately in FIG. 2 for clarity. L2 149 and C6151 is a high frequency post filter for reducing switching frequencyripple at the DC output terminals.

FIG. 2 shows one embodiment of a current and voltage sense circuit 147with voltage drop compensation in accordance with the teachings of thepresent invention used in the power supply of FIG. 1. The DC inputterminals are coupled across C5 145 on the secondary winding 129 side oftransformer T1 111 of FIG. 1. The DC output terminals are coupledthrough the high frequency post filter (L2 149 and C6 151 ) and a cable105 having a resistance RC1 107 and RC2 109, on positive and negativerails respectively, to the load 103. A high frequency post filter (L2149, C6 151) that is shown in FIG. 1 has very little effect, if any, onthe DC voltages and currents that are passed though directly to thecable 105. If L2 149 in FIG. 1 introduces any significant seriesresistance, this could be lumped with the cable resistances RC1 107 forvoltage drop compensation purposes. So, for the discussion that follows,the post filter is assumed to be transparent to DC voltages andcurrents.

In one embodiment, R5 153, R6 155, D7 157 and optocoupler LED 159constitute the current sense circuit. The voltage sense circuit includesPNP transistor Q1 161, R8 163, R9 165 and zener D8 167. In oneembodiment, zener D8 167 provides a voltage reference for the voltagesense circuit. The voltage drop compensation for the voltage sensecircuit is performed by R3 169, R4 171, D9 173 in conjunction with R8163.

In the embodiment shown, R6 155 and D7 157 are the current sensingcomponents in return path of the current on the negative rail coupled tothe DC output. The current through R6 155 and D7 157 is substantiallyequal to the load 103 current I_(L), which is supplied from the DCoutput and flows through the load 103 coupled to the DC output. Thevoltage dropped across R6 155 and D7 157 is applied across the LED 159of the optocoupler 131 through the resistor R5 153, which limits the LED159 current during the constant current mode of operation. R5 alsolimits the current that would flow through R6 155 and D7 157 when in theconstant voltage mode of operation. When the output current exceeds thecurrent sense threshold that is set by the difference between theforward voltages of the LED 159 and the PN junction diode D7 157, whichin one embodiment is approximately 300 mV, divided by the value of theresistance R6 155, the LED 159 of the optocoupler 131 conducts. This inturn reduces the power delivered by the power supply controller 125 tothe transformer 111 to maintain the output current in regulation at thecurrent sense threshold. In the example shown in FIG. 1, the estimatedvalue of R6 155 required for the constant current output of 600 mA is300 mV/600 mA, which equals 0.5 ohms. It is noted that the value of 0.51ohms for R6 155 shown in FIG. 1 was determined empirically for optimumpositioning for one embodiment for the constant current value within thespecification limits.

In the constant voltage mode of operation of one embodiment, the outputvoltage is regulated at a sense voltage that is the sum of the zener D8167 voltage, base emitter voltage V_(BE) of Q1 161, which isapproximately 0.7V, and any voltage drop across resistor R8 163 that isgenerated by the voltage drop compensation circuit. R9 165 providesconstant bias current through the zener D8 167 to reduce its dynamicimpedance. When the output exceeds the sense voltage, the transistor Q1161 conducts and drives the LED 159 of the optocoupler 131 to reduce thepower delivered by the power supply controller 125 to the transformer111 to maintain the output voltage in regulation at the sense voltage.

In one embodiment, R3 169 is the compensation resistor that is on thepositive rail coupled to the DC output and has a current that issubstantially equal to load 103 current I_(L). The voltage developedacross R3 169 is substantially equal to the voltage across the resistorR4 171 independent of temperature because the forward voltage drop ofdiode D9 173 and the V_(BE) of the transistor Q1 161 and theirtemperature coefficients are substantially the same in one embodiment.This is because both are PN junctions. Therefore, the current generatedby R4 171 is proportional to the load current I_(L) and flows through R8163. The base current of Q1 161 by design is negligible compared to thecurrent through R4 171. Therefore, the voltage drop across R8 163 isproportional to the load 103 current I_(L). As load 103 current I_(L)increases, the voltage drop across R8 163 increases, increasing thereference voltage and therefore the output voltage of the power supply101 to compensate for the cable 105 resistance drop to maintain asubstantially constant voltage V_(L) at the load 103 that is independentof the load 103 current I_(L).

In the example shown in FIG. 1, the R3 169 value was chosen to drop avoltage at full load of 600 mA that is substantially larger than anycomponent to component variation of differences in the diode D9 173forward voltage and the V_(BE) of Q1 161. In one embodiment, thecomponent voltage drop variations for PN junction diodes is in the orderof 20 mV. Accordingly, assuming that a voltage drop 10 times larger ischosen (e.g. 200 mV), the value of R3 169 is 200 mV/600 mA, which equals0.33 ohms. This value also has to be sufficient to provide the requiredvoltage drop compensation at full load. An R4 171 value of 120 ohms ischosen to provide a current as a function of load 103 that is muchlarger than the base current of Q1 161. At full load the current throughR4 171 is 200 mV/120 ohms, which is equal to 1.67 mA. This means the R4171 current will range from close to 0 mA at no load to 1.67 mA at fullload. The value of R8 163 is then chosen to provide sufficient drop atfull load to increase in sense voltage by an amount that is sufficientto compensate for the resistive voltage drops in the cable 105 coupledto the DC output. In the example of FIG. 1, the R8 163 value chosen is330 ohms, which changes the sense voltage by 1.67 mA×330 ohms, whichequals 0.55 V. This will provide compensation for a cable 105 resistanceof approximately 0.55V/600 mA, which equals 0.9 ohms. The calculationsabove neglect the change in zener D8 167 voltage with current due to itsinternal impedance, which is usually small compared to 330 ohms. In anycase, the actual value of R8 163 can be fine tuned empiricallyaccounting for second order effects of the circuit to provide asubstantially constant voltage output from no load to full load asmeasured at the load.

In the example shown, the collector current of Q1 161 is the sum of LED159 current, which in one embodiment is approximately 250 uA for drivingpower supply controller 125 devices independent of loading, and thecurrent through R5 153. The current though R5 153 is maximum at no loadand it can be estimated by dividing the voltage difference between theforward drop of the LED159 of the optocoupler 131 and the PN junctiondiode D7 157 by the resistor value of R5 153, which is: 300 mV/100 ohms,which equals 3 mA. This is because the drop across the resistor R6 155is negligible at no load. The total maximum Q1 161 current in oneembodiment is 3.25 mA. Assuming a beta for Q1 161 of 100, the basecurrent of Q1 161 is approximately 33 uA which translates to 33 uA×330ohms, which equals 10 mV across R8 163 at no load. This is negligiblecompared to the output voltage of 5.7V. A standard 4.7V zener diode D8167 is biased close to 5V by the current through R9 165 of 0.7V/180ohms, which equals 3.9 mA to center the output voltage at5V+V_(R8)(0V)+V_(BE) (0.7V), which equals 5.7V at no load.

In another embodiment, it is possible to eliminate the resistor R8 163by using zener diode D8 167 internal impedance to perform the functionof R8 163. In this embodiment, the value of R4 171 can be adjusted toprovide the required change in zener D8 167 current to provide thechange in sense voltage at full load.

In another embodiment, it is also possible to eliminate D9 173 and use acorrespondingly larger value of R4 171 to save cost. The voltage acrossR4 171 will then be equal to the drop across R3 169 plus a V_(BE)(approximately 0.7V at room temperature) and the value of R4 171 can bechosen to provide the desired amount of voltage compensation at roomtemperature. The voltage across R4 171 will vary with temperature due tothe V_(BE) temperature coefficient of approximately −2 mV/° C. This willchange the voltage compensation as a function of temperature that may beacceptable in some cases and desirable in some others.

FIG. 3 shows one embodiment of the IV (current vs. voltage)characteristics 301 at the load for the circuit in FIG. 1. The dottedline shows how the load voltage V_(L) would drop with increasing loadcurrent, due to the cable resistance, if there were no voltage dropcompensation.

FIG. 4 shows another embodiment of a circuit 447 in accordance with theteachings of the present invention that is a complementary version ofthe circuit 147 in FIG. 2. This circuit uses a NPN transistor Q1 461instead of a PNP transistor Q1 161 used in FIGS. 1 and 2. This requiresthat the current sensing is done on the positive rail coupled to the DCoutput and the voltage drop compensation is done on the negative rail.The component numbers shown in FIG. 4 match with the equivalentcomponents shown in FIG. 2. The operation and performance of thecircuits are otherwise identical to the circuit in FIG. 2.

FIG. 5 shows another embodiment of a novel current sense, voltage senseand voltage drop compensation circuit 557 according to the teachings ofthe present invention. In the embodiment in FIG. 5, the DC inputterminals are coupled across capacitor C5 145 on the secondary winding129 side of transformer T1 111 of FIG. 1. The DC output terminals arecoupled through a high frequency post filter (L2 149 and C6 151) and acable 105 having a resistance RC1 107 and RC2 109 as shown in FIG. 1, onpositive and negative rails respectively, to the load 103. In oneembodiment, R1 575, R2 577, D1 579 and the optocoupler LED 581constitute the current sense circuit. The voltage sense circuit includesR3 583, D2 585 and the optocoupler LED 581. The voltage compensationcircuit includes Q1 587 and R4 589. The operation of the current sensecircuit is identical to that described in FIG. 1.

In the constant voltage mode of operation of one embodiment, the outputvoltage is regulated at a sense voltage that is the sum of the voltageacross zener D2 585, the optocoupler LED 581 forward voltage drop andthe voltage across R3 583 minus the voltage across R1 575 and D1 579forward voltage drop. R2 577 limits the current through the optocouplerLED 581 when it is being driven by the current sense circuit (constantcurrent mode). R2 577 also provides bias current for zener D2 585 whenthe circuit is in a voltage sensing mode or constant voltage mode.

The voltage drop compensation is achieved by increasing the currentthrough R3 583 in response to increased output load 103 current I_(L).This function is achieved through the coupling of the base emitterjunction of Q1 587 in series with R4 589, in parallel to R1 575 and D1579. The voltage drop across R1 575 is representative of the output load103 current I_(L). The voltage drop across D1 579 is substantially equalto the base emitter drop of Q1 587 and has the same temperaturecoefficient providing temperature compensation. The voltage across R1575 is therefore substantially equal to the voltage across R4 589independent of temperature. The value of R4 589 therefore determines theQ 587 collector current for a given output load 103 current I_(L) andhence the relationship between the voltage drop across R3 583 and theoutput load 103 current I_(L). R4 589 is chosen to compensate for boththe voltage drop across R1 575 and the voltage drop due to cable 105resistance, RC1 107 and RC2 109, of FIG. 1.

In another embodiment, a complimentary version of the circuit shown inFIG. 5 is constructed. This is similar to the complimentary versions ofthe same circuit shown in FIGS. 2 and 4. In particular, thecomplementary circuit embodiment uses a PNP transistor instead of an NPNtransistor Q1 587 and the current sensing done on the positive railcoupled to the DC output. The voltage sense circuit would couple theoptocoupler LED to the positive output rail in series with D2 585 and R3583 coupled to the negative output rail with R4 589 coupled between thepositive rail and the PNP transistor emitter. The operation andcomponent choice is identical to the circuit of FIG. 5 described above.

In the foregoing detailed description, the method and apparatus of thepresent invention has been described with reference to specificexemplary embodiments thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

What is claimed is:
 1. A circuit, comprising: a current sense circuitincluding a first diode, a first resistor coupled to the first diode anda light emitting diode (LED) coupled to the first diode and the firstresistor, the LED is driven in response to a voltage drop across thefirst diode and the first resistor to sense a first current directedthrough the first diode and the first resistor; and a voltage sensecircuit coupled across an output of the circuit, the voltage sensecircuit including a transistor coupled to the LED; the transistorfurther coupled to a voltage reference, the LED further driven by thetransistor in response to a voltage across the output of the circuit. 2.The circuit of claim 1 wherein the transistor is coupled to drive theLED in response to the voltage across the output of the circuitexceeding a sense voltage of the voltage sense circuit.
 3. The circuitof claim 2 wherein the transistor comprises a bipolar transistor havinga base coupled to the voltage reference, the bipolar transistor to drivethe LED when the voltage across the output exceeds the sense voltage ofthe voltage sense circuit.
 4. The circuit of claim 3 wherein the voltagereference is provided by a zener diode coupled to the output of thecircuit.
 5. The circuit of claim 4 wherein the sense voltage includes asum of the voltage reference provided by the zener diode and a forwardbase emitter voltage of the bipolar transistor.
 6. The circuit of claim1 wherein the LED is included in an optocoupler.
 7. The circuit of claim6 wherein the optocoupler is coupled to provide feedback from an outputof a power supply, the optocoupler coupled to provide the feedback to apower supply controller included in the power supply.
 8. The circuit ofclaim 7 wherein the power supply is a switched mode power supply.
 9. Thecircuit of claim 1 wherein the first diode comprises a PN junctiondiode.
 10. A circuit, comprising: a voltage sense circuit coupled acrossan output, the voltage sense circuit including a voltage referencecoupled to the output to drive a base of a bipolar transistor coupled tothe output when a voltage across the output exceeds a sense voltage ofthe voltage sense circuit; and a compensation circuit including a firstresistor coupled to the output and the bipolar transistor, thecompensation circuit further including a second resistor, the secondresistor coupled between the first resistor and the base of the bipolartransistor such that a sum of a voltage drop across the first resistorand a forward base emitter voltage of the bipolar transistor is to beapplied across the second resistor, the sense voltage of the voltagesense circuit varied in response to a current through the secondresistor.
 11. The circuit of claim 10 further comprising a lightemitting diode (LED) coupled to be driven by the bipolar transistor, theLED included in an optocoupler.
 12. The circuit of claim 11 wherein theoutput is an output of a power supply, the optocoupler coupled toprovide feedback from an output to a power supply controller included inthe power supply.
 13. The circuit of claim 23 wherein the power supplyis a switched mode power supply.
 14. The circuit of claim 10 wherein thevoltage reference is provided by a zener diode coupled to the output ofthe circuit such that the current through the second resistor is passedthrough the zener diode, the sense voltage includes a sum of the voltagereference provided by the zener diode and the forward base emittervoltage of the bipolar transistor.
 15. The circuit of claim 14 furthercomprising a third resistor coupled to zener diode such that the currentthrough the second resistor is further passed through the thirdresistor, the sense voltage including a sum of the voltage referenceprovided by the zener diode, a voltage drop across the third resistorand the forward base emitter voltage of the bipolar transistor.
 16. Thecircuit of claim 10 wherein the compensation circuit further includes adiode coupled to the second resistor, the second resistor and diodecoupled between the first resistor and the base of the bipolartransistor such that a sum of the voltage drop across the first resistorand the forward base emitter voltage of the bipolar transistor is to beapplied across the second resistor and the diode.
 17. The circuit ofclaim 16 wherein the diode comprises a PN junction diode.
 18. Thecircuit of claim 10 wherein the sense voltage is coupled to be increasedin response to an increase in a load current supplied from the output.19. A circuit, comprising: a voltage sense circuit coupled across anoutput, the voltage sense circuit including a first resistor, a zenerdiode coupled to the first resistor and a light emitting diode of anoptocoupler coupled to be driven by the first resistor and the zenerdiode when a voltage across the output exceeds a sense voltage of thevoltage sense circuit, the sense voltage of the voltage sense circuitincludes a sum of a voltage drop across the zener diode, a forwardvoltage of the LED and a voltage drop across the first resistor; acurrent sense circuit coupled to the output, the current sense circuitcoupled to provide a voltage representative of a load current suppliedfrom the output; and a voltage compensation circuit coupled to thecurrent sense circuit and to the voltage sense circuit to vary thevoltage drop across the first resistor in response to the voltagerepresentative of a load current supplied from the output.
 20. Thecircuit of claim 19 wherein the voltage drop across the first resistoris increased in response to an increase in the load current suppliedfrom the output.
 21. The circuit of claim 19 wherein the output is anoutput of a power supply, the optocoupler is coupled to provide feedbackfrom the output to a power supply controller included in the powersupply.
 22. The circuit of claim 21 wherein the power supply is aswitched mode power supply.
 23. The circuit of claim 19 wherein thecurrent sense circuit includes a second resistor coupled to a diode, thesecond resistor and diode coupled to the output such that the loadcurrent is directed through the second resistor and the diode.
 24. Thecircuit of claim 23 wherein the voltage compensation circuit includes athird resistor coupled to an emitter of a bipolar transistor, a seriescombination of the third resistor and a base emitter junction of thebipolar transistor coupled across the current sense circuit such that acollector current of the bipolar transistor is proportional to a voltagedrop across the second resistor.
 25. The circuit of claim 24 wherein acollector of the bipolar transistor is coupled to the first resistorsuch that the voltage drop across the first resistor is responsive tothe voltage drop across the second resistor.
 26. A circuit, comprising:a current sense circuit coupled to an output of the circuit, the currentsense circuit including, a first diode; a first resistor coupled to thefirst diode, the first resistor and first diode coupled to the outputsuch that a load current supplied from the output is directed throughthe first diode and the first resistor; a second resistor coupled to thefirst resistor and the first diode; and a light emitting diode coupledto the first resistor and the second resistor and the diode such thatthe voltage drop across the first resistor and the first diode iscoupled to drive the LED; a voltage sense circuit coupled to the outputand the current sense circuit, the voltage sense circuit including, azener diode coupled to the output; a third resistor coupled in serieswith the zener diode; a fourth resistor coupled to the zener diode andthe output; and a bipolar transistor coupled to the LED, the output andthe third resistor, the bipolar transistor coupled to drive the LED whena voltage across the output exceeds a sense voltage, the sense voltageincluding a sum of a voltage drop across the zener diode and a forwardbase emitter voltage of the bipolar transistor; and a voltagecompensation circuit coupled to the output and the voltage sensecircuit, the voltage compensation circuit including, a fifth resistorcoupled to an emitter of the bipolar transistor and to the output suchthat a current through the fifth resistor is substantiallyrepresentative of the load current supplied from the output; a sixthresistor coupled to the fifth resistor a second diode coupled to thesixth resistor, the sixth resistor and the second diode coupled inseries between a base of the bipolar transistor and the fifth resistorsuch that the sense voltage is varied in response to the current throughthe fifth resistor.
 27. The circuit of claim 26 wherein the bipolartransistor includes one of a PNP transistor or an NPN transistor.
 28. Acircuit, comprising: a first diode; a first resistor coupled to thefirst diode, the first resistor and first diode coupled to an output ofthe circuit such that a a load current supplied from the output isdirected through the first diode and the first resistor; a secondresistor coupled to the first diode and the first resistor; a lightemitting diode (LED) coupled to the first resistor and the secondresistor and the diode such that the voltage drop across the firstresistor and the first diode is coupled to drive the LED; a thirdresistor coupled to the output; a zener diode coupled to the thirdresistor and to the LED; a bipolar transistor having a collector coupledto the third resistor and the zener diode and a base coupled to theoutput; and a fourth resistor coupled between an emitter of the bipolartransistor and the first resistor.
 29. A method, comprising: sensing avoltage across an output of a circuit with a voltage sense circuithaving a sense voltage that is the sum of a voltage reference and a baseemitter voltage drop of a transistor; regulating the voltage across theoutput of the circuit at the sense voltage in a closed loop;compensating a voltage drop in a resistor that couples the output of thecircuit to a load using a compensation circuit; changing the sensevoltage with the compensation circuit as a function of a load currentsupplied by the output of the circuit to compensate for the voltage dropacross the resistor; and canceling substantially the base emittervoltage drop of the transistor with a diode included in the compensationcircuit.
 30. The method of claim 29 further comprising: sensing the loadcurrent with a current sense circuit having a current sense threshold;regulating the load current at the current sense threshold in a closedloop when the load current reaches the current sense threshold.